The present invention relates to the art of encoding the movement of a web. More particularly, the invention relates to devices for encoding the motion of a photoreceptor belt in an electrophotographic, or xerographic printing apparatus.
The invention is particularly applicable to encoding the movement of a continuous photoreceptor belt in a multipass, multichromatic (multicolor) electrophotographic printing device and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications, such as encoding the position of a moving web in environments besides those that involve electrophotographic printing.
Electrophotographic printing involves the use of a photoconductive member that is initially charged to a substantially uniform potential. An electrostatic latent image is formed on the photoconductive member, usually by way of a raster output scanner (ROS), which discharges the charged photoconductive member in selected areas. The latent image is then developed by bringing a developer material, typically a toner powder, into contact with the surface. The developed image is then transferred to a copy sheet and permanently affixed thereto by fusing in a heating device.
In multicolor printing, a plurality of images are recorded and developed on the photoconductive member, which usually takes the form of a continuous belt. Typically, a four-color image requires a separate image for each of four colors, i.e., black, cyan, magenta, and yellow, which are recorded on the photoreceptor belt and later superimposed to form a single image on the recording medium.
In single pass color printing, the color separations are superimposed on the photoreceptor belt before being transferred to the recording medium. The photoreceptor belt thus makes only a single pass to acquire and develop the latent images for each of the color separations and transfers a multicolor image to the recording medium in a single operation.
In multipass color printing, one color separation is imaged and developed on the photoreceptor belt and transferred to the recording medium before the next color separation is imaged, developed and transferred. Thus, each color separation is transferred to the recording medium before the next one is developed, imaged and transferred. Thus, the photoreceptor belt makes multiple passes to transfer a given multiple color image to a sheet of the recording medium.
Both single and multipass color printing require precise control of the photoreceptor belt and its interaction with the imaging, developing and transfer stations of the printing apparatus in order to achieve the correct registration between the color separations and to avoid any image degradation. The motion of the photoreceptor belt must be accurately controlled, especially in the span of the belt which encompasses the imaging and developing stations. The positional accuracy required for acceptable registration in the trade is typically below a maximum limit of 125 micrometers. Some imaging techniques require registration accuracy of no more than 15 micrometers between color separations for pictorial information.
Various devices and systems for controlling and synchronizing photoreceptor belt motion are known. For example, U.S. Pat. No. 5,200,782 discloses a color printing device which utilizes an encoding roller to track the motion of the photoreceptor belt. The encoder provides belt motion and registration information to a servomechanism that controls the belt drive roller. The encoder can also provide motion information to the writing heads that generate the latent images on the belt. Similarly, U.S. Pat. No. 5,200,791 discloses a color registration system that utilizes an encoder roller to provide a clocking signal for controlling color registration. U.S. Pat. No. 5,153,644 discloses a xerographic system which incorporates an encoder wheel on the photoreceptor belt. The wheel is situated on the top of the photoreceptor belt and a backing roller is provided on the underside of the belt to support the same. The encoder wheel is positioned at one edge of the belt.
Encoder rollers typically comprise an elongate roller that extends across and engages the span of the photoreceptor belt. The roller shaft is connected to an encoding device that generates an electronic encoder signal corresponding to the roller rotation and belt speed. In order for the encoder signal to accurately control the belt speed, the roller eccentricity and composite runout must be kept within very strict tolerances. Eccentricity refers to the variation between the rotational center and the geometric center of the roller. Composite roller runout refers to the overall variation in eccentricity across the length of the roller. Since the roller speed control system operates in closedloop fashion to maintain encoder roller angular velocity constant, roller eccentricity and runout result in small variations, or modulations, in the linear velocity of the PR belt. This will contribute ultimately to registration errors.
Some known electrophotographic printing devices incorporate an encoder roller that operates synchronously with the photoreceptor belt. The belt length is selected as an integer multiple of the encoder roller circumference such that, ideally, the encoder roller is in the same phase orientation with every once-around of the photoreceptor belt. In such devices, the roller runout must be carefully controlled in order to maintain synchronous operation and keep color registration within acceptable limits. Acceptable composite runout tolerances are typically within .+-.0.05 mm. On a long roll, such tolerances become difficult to maintain and result in increased manufacturing costs. Thus, providing a low cost encoder roller with acceptable accuracy has heretofore presented a problem.
Applicants have found that, in printing devices, especially multipass architectures which use a synchronous encoder roller and photoreceptor belt, roller diameter and eccentricity are the two largest contributors to process direction misregistration. It is advantageous to provide an increased roller diameter with minimal eccentricity and composite runout. However, space limitations within most printing devices prevent the use of large diameter encoder rollers. This is typically due to the presence of other hardware beneath the belt span. Thus, providing an encoding device that accomplishes the aforementioned objectives has heretofore presented a problem.